Autotransformer tap question

[Leo_Gnardo]

Unless the schematic is wrong, there is no mains connection to chassis. Even in hot chassis amps, we don't usually see a direct connection to chassis, that is usually done with a cap.

OK, then the OP should check with an ohm meter to make sure there is no connection, that line-to-chassis cap hasn't shorted, or that some bright spark hasn't added a wire. He DID mention there was a line connection to chassis. I'm glad to see that such a thing doesn't show up on the screamatic. Let's verify that on this AKAI so nobody gets a shock or worse.

This is two amp chassis with power transformers, then an autoformer to feed them. No different from plugging an amp into your variac.

Kee-rect.

I missed the reason we need to get rid of the voltage selector. If it is intact and works, why remove it?

Only for simplification. You're right Enzo: not necessary to remove. Let's revisit post #6 ^^^^^^, kinda got lost behind the schemo download discussion.

The committee is inching toward a solution, and that's good!

[paggerman]

Hi, thanks for the reply. What you say makes more sense than what I can make from the original wiring. The chassis is not hot. The output from the autotransformer goes to the primary of the PT in the 'safe' way. The valves are 12ax7, ef86 and a pair of el84 with standard 6.3v heater wiring. I need to add an earth connection to the chassis.

I have attached a photo of how I currently have the wiring. The 230v tap is open circuit (maybe something to do with the alfoil 'fuse' I found) so I have attached the mains hot/live to the 220v tap via a fuse. The neutral of the mains is connected to the 0v tap directly. I haven't plugged this in yet - I don't have confidence that I have it wired correctly. The red and green wires are on the 0v and 100v taps to feed into the chassis PT.

Although I have quite a bit of experience of building and repairing valve amps safely (most of my learning has been thanks to this forum) , I have not previously encountered an autotransformer and I don't want to take unnecessary risks.

Your advice is welcome. Thank you very much.

[paggerman]

Ah. The reason for not retaining the original wiring. First, the mains cable had a hidey hole built into the cabinet and I will need that cavity space for the speaker I will be installing in the cabinet (making it into a combo) and secondly the original wiring was plugged into the transport chassis whence further plug in connectors powered the two amplifier chassis. I wanted rid of all those connectors and intermediary stages.

[Tom Phillips]

...The 230v tap is open circuit (maybe something to do with the alfoil 'fuse' I found) so I have attached the mains hot/live to the 220v tap via a fuse. The neutral of the mains is connected to the 0v tap directly. I haven't plugged this in yet - I don't have confidence that I have it wired correctly. The red and green wires are on the 0v and 100v taps to feed into the chassis PT.,,

Your connection method is correct to obtain a 100V output from a 220V input. Too bad that the 230 V tap on the auto transformer tap is open circuit. If that was caused by over current then there could be some other latent damage and you will also want to find out what caused the original problem. Hopefully it was not a fault in the other transformer. The auto transformer will be working closer to saturation if you apply higher than 220V to the 220V tap. I’d say that a slight overage (up to 10%) isn’t normally a problem but you do already have a damaged part. Just something to think about.

Other comments:
1) You don’t show an On/OFF switch. Are you putting that off until after you prove that everything works?
2) Using the 220V input tap the 100V output will be ~105V if the actual line voltage is 230V and ~109V if the actual line voltage is 240V. This is probably no big concern. It’s pretty much the same situation we see with a lot of the old equipment running on modern line voltages. After you power up you can measure the actual voltages.
3) Your first post says that the PT also has a 117V primary tape. Therefore, you could consider using that with the “110V” output tape on the auto transformer. The 110V tap will produce 115V with a 230V line input and 120V with a 240V line input with your proposed setup. Actual voltage measurements will give you the data you need to decide.

Cheers,
Tom

[paggerman]

Very many thanks to everybody. I will plug in (via light bulb) tomorrow and measure the outputs of the autotransformer. There is a power switch on the chassis, but it's not a problem to pop one in on the mains side of the autotransformer. Cheers.